Currently, an ever increasing need for careful management of energy exists in the electric/electronic, transportation and other fields. For more precise control, the system is equipped with more than ever electronic components. In the transportation field, for example, a transition takes place from gasoline vehicles to hybrid vehicles, plug-in hybrid vehicles, and electric vehicles. These hybrid and electric vehicles must be loaded with motors, inverters, batteries, and other electronic components which are unnecessary for gasoline vehicles. Nowadays, heat-conductive silicone grease compositions become indispensable to conduct heat from these heat-generating electronic components to cooling units in an efficient way.
More than ever electronic components must be mounted within a limited space, indicating that electronic components are kept under widely varying conditions including temperature, mount angle, etc. Under the circumstances, heat-generating electronic components and heatsinks are not always held horizontal and accordingly, a heat-conductive material connecting them is often mounted at a certain angle. In such a service environment, a heat-conductive silicone adhesive material, heat-conductive potting material, or RTV heat-conductive silicone rubber composition is used in order to prevent the heat-conductive material from sagging and falling out of the space between the heat-generating component and the heatsink, as disclosed in JP-A H08-208993, JP-A S61-157569, JP-A 2004-352947, JP 3543663, and JP 4255287. However, all these heat-conductive materials form a complete bond to members and undesirably lack re-workability. Since the heat-conductive material becomes very hard after bonding, it cannot withstand repeated stresses induced by thermal strain and separates apart from the heat-generating component, leading to a ramp of thermal resistance. On curing, the heat-conductive material can cause stresses to the electronic component.
The above problem can be solved by a one package addition cure heat-conductive silicone composition as disclosed in JP-A 2002-327116. This composition remains re-workable and anti-sagging even after heat curing. In addition, the cured composition which is a relatively soft rubber may play the role of a stress relief agent. Nevertheless, this heat-conductive material suffers from several problems. For example, it must be stored in a refrigerator or freezer and thawed prior to use. In applying the heat-conductive silicone material, it must be heated and cooled. Then the manufacturing system must be equipped with a heating/cooling oven. The heating and cooling steps take a long time, leading to a reduction of manufacturing efficiency. From the standpoint of energy efficiency, these steps are inefficient because not only the heat-conductive material, but also an overall component must be heated. Additionally, there is a potential risk that if any metal cutting fluid containing an amine compound which is a cure inhibitor is left on the coating surface, the heat-conductive material remains under-cured.
To obviate the cumbersome handling of heat-conductive material including refrigeration/thaw management for storage and heating/cooling steps for application, JP-A 2003-301189 proposes a one package addition cure heat-conductive silicone composition which has been heat crosslinked during preparation. This heat-conductive silicone grease composition has overcome the above-discussed problems, but the tradeoff is that it has too high a viscosity to coat. There are problems that heavy loading of filler is difficult due to the high viscosity of the base polymer and the manufacture process involving crosslinking reaction takes a long time.
One development is a RT moisture-thickening heat-conductive silicone grease composition of dealcoholization type which is storable at RT, which has a low viscosity enough to coat at the initial, and which after coating, increases its viscosity with moisture at RT so that it becomes anti-sagging. Since this silicone grease composition is designed to increase its viscosity (or thicken) with moisture rather than curing with moisture, it is re-workable and applies no substantial stresses to the electronic component. The composition is expected to exploit a new application for heat-conductive silicone grease. However, the composition was found to lack high-temperature durability. There is an urgent need to address the lack of durability.